The invention relates to a low differential pressure syphon assembly for steam heated rotary dryer drums and particularly pertains to a pick-up shoe for a rotating syphon assembly.
The condensate removal means requirements in a high speed rotating drum application differ from those of stationary or low speed rotating drum applications. In the art of steam dryer drums for paper machines and the like, syphon pipes have long been used to remove the condensate from the drum and means to aid condensate pick-up such as scoops and flow directional means are commonly used with stationary syphon pipes where the condensate is removed from the lowest part of the drum. Rotating syphons are used in applications where due to the higher rotational speed of the drying drum, the condensate is distributed in a more or less uniform layer on the entire interior circumference of the drying drum because of centrifugal force. In these high rotational speed applications, the centrifugal force also frustrates condensate removal from the drum interior wall.
Typically, a steam heated rotary syphon handles both a vapor and a fluid; the vapor being inadvertently introduced into the syphon as the condensate is drawn into the syphon. The pressure loss characteristics of a conventional rotary syphon do not exhibit a monotonic behavior with the vapor mass flow. At high flow rates the vapor mass flow causes a pressure loss which increases quadratically as the flow increases. On the other hand, at low flow rates, the density of the mixture increases and a pressure loss occurs in lifting the mixture through the syphon assembly to the rotational axis of the drum against the centrifugal force acting on the condensate. Also, if the condensate flow ceases for a sufficient period, the fluid level inside the roll will increase and the syphon entrance will be covered necessitating a large differential pressure to move a dense fluid column.
Syphons aspirated with steam have been used to augment condensate removal and United States patents disclosing aspirated rotating syphons include U.S. Pat. Nos. 4,718,177; 4,606,136; 4,516,334; 2,993,282 and British specification No. 2,413,271 also discloses another example of an aspirated syphon. Each of the aforementioned patents incorporate a flow directing means within a pick up tip unlike the invention's novel open condensate flow path design. The pressure losses attendant to designs of this type require the application of a compensatorally higher differential pressure to remove the condensate from the drum.
U.S. Pat. No. 4,718,177 employs a flow directional means in its design in the form of a steam blow line with a triangular cross-section. This patent combines the redirecting effect of the triangle, the base of which is parallel to the longitudinal axis of the drying drum and a jet pump effect is accomplished through the injection of steam into and in the direction of condensate flow from a plurality of holes in the apex of the redirecting device positioned in an elongated slot in the cylindrical condensate pickup means.
Some conventional steam aspirated syphons incorporate a small vapor injection port, typically, on the order of 0.05 square inches located at a radial position displaced toward the cylinder centerline from the syphon tip. These patents have vapor introduced at such a location that it does not act on the drum wall fluid interface and, consequently, is much less effective in assisting the fluid discharge. U.S. Pat. Nos. 2,993,282, 4,606,136 and the British disclosure No. 241,171 are of this manufacture. With these devices the steam injection port does not convey vapor into and counter to the condensate flow for reducing the mixture density. Mixture density reduction enhances condensate removal efficiency which is especially important to offset the increase in pressure loss at low flows. Another inherent limitation of small holes is that they offer minimal assistance in reducing the mixture density if the tip is covered by condensate.
An object of the invention is to provide a rotating dryer drum syphon adapted to more effectively remove the condensate at lower differential pressures than conventional rotary syphons.
Another object of the invention is to provide a rotary syphon which prevents flooding of the steam ports within design limits of the invention.